Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging, and particularly to a three-dimensional magnetic resonance imaging method and system.
Description of the Prior Art
Magnetic resonance imaging (MRI) is an imaging technology using magnetic resonance phenomenon. The principle for magnetic resonance phenomenon mainly includes: the protons of atomic nuclei containing an odd number of protons such as the nuclei of hydrogen atoms which are widely present in the human body have spin motion like small magnets, and the spin axes of these small magnets have no definite law; if an external magnetic field is exerted, these small magnets would be rearranged according to the line of magnetic force of the external magnetic field, particularly arranged in the two directions respectively parallel to or anti-parallel to the line of magnetic force of the external magnetic field, in which the above-mentioned direction parallel to the line of magnetic force of the external magnetic field is termed as a positive longitudinal axis while the above-mentioned direction anti-parallel to the line of magnetic force of the external magnetic field is termed as a negative longitudinal axis; the atomic nuclei only have longitudinal magnetized vectors, in which the longitudinal magnetized vectors have both direction and amplitude. The atomic nuclei in an external magnetic field are subjected to a pulse excitation using a radio frequency (RF) of a specific frequency so as to enable the spin axes of these atomic nuclei to deviate from the positive longitudinal axis or the negative longitudinal axis to produce resonance, which is magnetic resonance phenomenon. The atomic nuclei would have transverse magnetized vectors after the spin axes of the above excited atomic nuclei deviate from the positive longitudinal axis or the negative longitudinal axis.
The absorbed energy is gradually released in the form of electromagnetic waves as the excited atomic nuclei emit echo signals after the radio frequency pulse emission stops, then both the phase and energy level of the excited atomic nuclei recover to the states prior to excitation, and an image can be reconstructed after further processing, e.g. space encoding the echo signals emitted from the atomic nuclei.
In the prior art, the dual echo steady state (DESS) sequence has obvious advantages on the imaging of complex anatomical structures, especially in a three-dimensional magnetic resonance image, and the isotropic high-resolution voxels of the three-dimensional magnetic resonance image produced by the dual echo steady state (DESS) sequence are used to allow the image be able to be reformatted into any plane, thus facilitating the diagnosis. Unfortunately, the running time based on the dual echo steady state (DESS) sequence is approximately five minutes, while the relatively long scanning time would affect the degree of comfort of a patient.
FIG. 3 is a schematic diagram for the dual echo steady state sequence according to the prior art. As shown in FIG. 3, the dual echo steady state sequence according to the prior art comprises a second radio frequency pulse RF2 and a second gradient pulse, in which the duration among the various second radio frequency pulses RF2 is the second repetition time TR2, the second gradient pulse comprises a second readout gradient pulse Gr2, a second layer selection gradient Gs2 and a second phase gradient (not shown); the second readout gradient Gr2 further comprises a second predispersion phase gradient PrePhase2, a second platform gradient RO2 and a second redispersion phase gradient RePhase2, in which the duration and the amplitude of the first predispersion phase gradient PrePhase1 are equal to those of the first redispersion phase gradient RePhase1, and therefore the areas (equal to duration multiplied by amplitude) thereof would also be equal.
As shown in FIG. 1, during the second platform gradient RO2, the magnetic resonance imaging system collects two symmetric echoes (or referred to as one full echo), i.e., the second free induction decay (FID) signal FID2 and the second echo signal Echo2. As shown in FIG. 3, the second repetition time TR2 partially depends on the duration of the two symmetric echoes (or referred to as one full echo).
In addition, as shown in FIG. 3, the latter second echo signal Echo2 is the refocused signal of the former second free induction decay (FID) signal FID2, and therefore the latter second echo (Echo) signal Echo2 keeps the information of the former second free induction decay (FID) signal FID2.
A dual echo steady state (DESS) sequence of magnetic resonance imaging and the calculation method for the acquisition time thereof are introduced in detail in the patent application document with the publication No. CN 1499218 A, belonging to Siemens Ltd. In the prior art, time is saved by means of coil sensitivity information and using parallel acquisition technique, GRAPPA undersampling technique or CAIPIRINHA undersampling technique and the like.